The present invention concerns electro-optical sensors operating in the middle ultra-violet (UV) region of the electromagnetic spectrum, and more specifically in the so called "solar blind" region, namely the range between 230 nm and 280 nm wavelengths. Such sensors are generally known, mostly used for the detection of flames e.g. forest fires and artificial light sources.
The name "solar blind" derives from the known fact that the ozone layer in the stratosphere completely absorbs the solar radiation of wavelengths below 280 nm - as represented by the most lefthand shaded area in the Solar Spectral Irradiance at sea level curve of FIG. 1a. The lower limit of 230 nm is determined by the edge of the Shumann-Runge oxygen absorption band.
Even though it is well known that conventional electro-optical "solar blind" sensors have an operational advantage over sensors operating in longer wavelength ranges, in that even under the most stringent daylight conditions their operation remains largely unaffected by background noise, it is the major object of the present invention to provide a sensor whose responsivity is boosted to a signal sensitivity of a single photon per second, thereby considerably improving its performance.